annual report annual report annual report annual report 2005

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RADIATION CHEMISTRY AND PHYSICS, RADIATION TECHNOLOGIES
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DENSITY FUNCTIONAL THEORY STUDY OF Na +···• CH 3 COMPLEX
STABILIZED IN DEHYDRATED Na-A ZEOLITE
Marek Danilczuk 1,2/ , Dariusz Pogocki 1/ , Monika Celuch 1/
1/
Institute of Nuclear Chemistry and Technology, Warszawa, Poland
2/
Department of Chemistry and Biochemistry, University of Detroit Mercy, Detroit, USA
It is well-known that the porous nature of zeolite
plays an important role in the separation and stabilization
of various reactive intermediates as atoms,
radicals, radical ions and metallic clusters [1,2].
Studies of these intermediates are very important
for better understanding of the mechanisms of chemical
reactions in heterogeneous catalysis as well as
for photochemistry, radiation chemistry and environmental
applications [3-5].
Recently, metal loaded zeolites were found to
be promising catalysts in the decomposition of automotive
and power plants exhaust emission. Synthetic
zeolites play an important role in petrochemistry
enabling catalytic conversion of hydrocarbons
to liquid fuels.
One of the relatively simple, yet very important
species stabilized in zeolites is methyl radical, for
which its lifetime in the zeolites framework is extended
to hours even at room temperature [6] compare
to µs in liquid hydrocarbons.
The adsorption/stabilization of methyl radicals
in zeolites has been studied by electron paramagnetic
resonance (EPR) spectroscopy, where isotropic
and anisotropic hyperfine coupling constants
have provided an important insight into the electron
structure/distribution in polyatomic radicals.
The observed EPR spectrum of methyl radicals in
zeolites usually exhibits isotropic quartet similar
to those observed in the liquid phase or inert gas
matrixes. However, line shape and finally hyperfine
coupling constants depend on the nature of
adsorption sites and activation of zeolite (dehydratation)
[7].
Application of quantum chemistry methods can
provide here important information on the nature
of radical-zeolite interaction, particularly of the
electronic and magnetic properties of the intermolecular
adsorption complex. For example, the
interpretation of hyperfine coupling constants of
carried out by theoretical calculations applying
even semiempirical methods can often help to rationally
explain the experimental results.
However, due to the complexity of adsorbates-
-zeolite systems, calculations hyperfine coupling
constants have been limited to rather simple systems
[8] do not mention that the application of
very sophisticated ab initio or density functional
theory (DFT) methods require here quite substan-
Fig. Two investigated models: a) 3T cluster and b) six-membered (6T) ring with sodium cation and methyl radical.
Geometrics optimized with B3LYP/LANL2DZ theory level.